Seventies rock revivalists Black Mountain have always dabbled with synthesizers, but they’ve never embraced them as much as they do on IV. The grooves are still epic and stony, but for every Sabbath-imbued jam, like the glorious, eight-minute opener, there’s a Pink Floyd-guided suite, like “Defector,” which mixes airy, bluesy vocals, icy keys, and an insistent, trudging bassline. Somehow, though, it’s the tighter tunes that are even more exploratory. On the gothy “Cemetery Breeding,” Stephen McBean and Amber Webber deliver crushingly gorgeous lines like "When you looked in my eyes/I was dreaming of suicide” between Echo & The Bunnymen-style synth-guitar riffs.